Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on a substrate such as a silicon wafer. A photolithography method is a means to form an integrated circuit pattern formed on a photomask, on a resist on a wafer by reduced projection by means of a projection optic system of an exposure machine. Along with miniaturization of patterns of semiconductor devices, as a source light of an exposure machine used at the time when an integrated circuit pattern on a photomask is transferred to a resist on a wafer, a shorter wavelength EUV light is prospective, rather than ArF excimer laser having a wavelength of 193 nm. EUV light is meant for a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for a light ray having a wavelength of from 5 to 20 nm, particularly 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional refractive system like photolithography employing visible light or ultraviolet light. Therefore, in EUV light lithography, a reflective system i.e. a combination of a reflective photomask and a mirror, is employed.
A mask blank is a stacked member before patterning, to be employed for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass. As the reflective layer, a multilayer reflective film having a high refractive index layer and a low refractive index layer alternately stacked is commonly used. In the multilayer reflective film, a plurality of interfaces between a high refractive index layer and a low refractive index layer are present, and a high reflectivity is obtained by adjusting the thicknesses of the high refractive index layer and the low refractive index layer so that phases of weak reflected light resulting at the respective interfaces are the same. Thus, the high refractive index layer and the low refractive index layer constituting the multilayer reflective film are required to have a regular periodic structure of predetermined thicknesses. Further, on the outermost surface of the multilayer reflective film, a protective layer having a resistant to etching for the absorber layer patterning is formed in some cases. In the present invention, in a case where a protective layer is formed on the multilayer reflective film, the multilayer reflective film including a protective layer will be referred to as a multilayer reflective film. For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing Cr or Ta as the main component, is used. On the outermost surface of the absorber layer, a low reflective layer having a low reflectivity for the light having a wavelength of inspection light to be used for inspection after patterning, is formed in some cases so that the pattern shape will easily be detected by an inspection after patterning.
As defects problematic in an EUV mask blank, phase defects and amplitude defects are present. Local irregularities such as pits, scratches and foreign matters present on the substrate surface and foreign matters included in the multilayer reflective film cause disorder in the periodic structure of layers of the multilayer reflective film deposited thereon, and locally change the phase of EUV light reflected on a photomask set to an exposure machine. If a photomask using an EUV mask blank having phase defects is set to an EUV exposure machine and a pattern on such a photomask is transferred to a wafer, a pattern transferred to the wafer is an incomplete pattern different from the desired pattern. Accordingly, the number of phase defects of a predetermined size or larger should be 0. Further, foreign matters in the vicinity of the surface of the multilayer reflective film and foreign matters included in the absorber layer have a negative effect to absorb EUV light and thereby to decrease the intensity of EUV reflected light from the multilayer reflective film. Also if a photomask using an EUV mask blank having amplitude defects is set to an EUV exposure machine and a pattern on such a photomask is transferred to a wafer, a pattern transferred to the wafer is an incomplete pattern different from the desired pattern. Accordingly, the number of amplitude defects of a predetermined size or larger should also be 0.
As a method for inspecting an EUV mask blank for phase defects and amplitude defects, a dark field inspection method using EUV light and a bright field inspection method or a dark field inspection method using DUV (deep ultraviolet) light (wavelength: 150 to 380 nm) or visible light (wavelength: 380 to 600 nm) may be mentioned. In the bright field inspection method, inspection light is applied to an object to be inspected and the intensity of the obtained reflected light is measured. The intensity of reflected light from a portion where a defect is present and the intensity of reflected light from a normal portion where no defect is present are different from each other, and the difference correlates with the size of the defect, and therefore, by utilizing the difference, the location where the defect is present and the size of the defect are specified. Whereas by the dark field inspection method, inspection light is applied to an object to be inspected and the intensity of obtained scattered light is measured. The intensity of scattered light from a portion where a defect is present and the intensity of scattered light from a portion where no defect is present are different from each other, and the difference correlates with the size of the defect, and therefore, by utilizing the difference, the location where the defect is present an the size of the defect are specified.
The dark field inspection method using EUV light is useful as a phase defect inspection method after formation of the multilayer reflective film. Non-Patent Document 1 discloses that by the dark field inspection method using EUV light after formation of the multilayer reflective film, a convex phase defect having a full width of half maximum of 36 nm and a height of 1 nm can be detected. Assuming that such convex defects have a Gaussian function shape, the sphere equivalent volume diameter (the diameter when the volume of a defect is calculated as a sphere, hereinafter referred to as SEVD) of the defect as calculated in accordance with Non-Patent Document 4 is 14 nm, and thus a phase defect of a very small size can be detected. However, it is known that the sensitivity for detection of an amplitude defect resulting from a foreign matter in the vicinity of the surface of the multilayer reflective film, by the dark field inspection method using EUV light, is lower than the sensitivity for detection of a phase defect resulting from a local irregularity present on the substrate surface or a foreign matter included in the multilayer reflective film. Specifically, Non-Patent Document 2 discloses that the minimum size of an amplitude defect which can stably be detected by the dark field inspection method using EUV light is 100 nm, which is 7 times the above-described detection limit size of a phase defect, and the sensitivity for detection of an amplitude defect tends to be insufficient.
Whereas, it is known that by the bright field inspection method or the dark field inspection method using DUV light or visible light, a phase defect and an amplitude defect can be detected at the same level of the sensitivity for detection. Specifically, Non-Patent Document 3 discloses that the minimum size of a phase defect which can stably be detected by the dark field inspection method at a wavelength of 266 nm is a convex defect having a full width of half maximum of 85 nm and a height of 2.7 nm. SEVD of such a defect obtained in accordance with the method disclosed in Non-Patent Document 4 is 35 nm. Whereas, the minimum size of an amplitude defect which can stably be detected by the dark field inspection method at a wavelength of 266 nm, as a result of evaluation using silica particles spread on the surface of the multilayer reflective film, is SEVD of 45 nm as reported in Non-Patent Document 3. Accordingly, the dark field inspection method at a wavelength of 266 nm is very superior in the sensitivity for detection of an amplitude defect to the dark field inspection method using EUV light, although it is inferior in the sensitivity for detection of a phase defect.
Accordingly, the dark field inspection method using EUV light is excellent in detectability of phase defects but is inferior in detectability of amplitude defects, whereas the bright field inspection method or the dark field inspection method using DUV light or visible light has detectabilities of amplitude defects and phase defects at the same level, but is inferior in the detectability of phase defects to the dark field inspection method using EUV light. Thus, there has been no method for stably detecting both phase defects and amplitude defects. To overcome such a problem, Patent Document 1 discloses a method for inspecting an EUV mask or an EUV mask blank for phase defects and amplitude defects by carrying out both dark field inspection method using EUV light and dark field inspection method using DUV light by an EUV mask inspection apparatus provided with both EUV light source and DUV light source.